1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in a magnetic read head, more specifically to a spin valve type of GMR sensor having a synthetic antiferromagnetically pinned (SyAP) layer.
2. Description of the Related Art
One of the most commonly used structural configurations of magnetic and non-magnetic layers in giant magnetoresistive (GMR) read-heads is the so-called spin-valve magnetoresistive (SVMR) structure. In the most basic version of the SVMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or xe2x80x9cpinned,xe2x80x9d by exchange coupling with an antiferromagnetic (AFM) layer, usually a layer of MnPt, directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can rotate its magnetization vector in response to small variations in external magnetic fields such as are produced by moving magnetic media, (which variations do not affect the magnetization direction of the pinned layer). The rotation of one magnetization relative to the other produces changes in the resistance (magnetoresistance) of the three layer structure. A constant current sent through the SVMR then produces voltage variations across it, which are sensed by external circuitry. More germane to the present invention are newer versions of the SVMR that utilize a so-called synthetic antiferromagnetically pinned (SyAP) layer which is a tri-layered lamination comprising two ferromagnetic layers separated by a thin metallic, non-magnetic layer and wherein the two ferromagnetic layers are magnetically oriented in antiparallel directions by exchange coupling. In the SVMR configuration, this SyAP pinned layer would then be pinned by an additional antiferromagnetic material (AFM) pinning layer. Methods for fabricating several versions of this SyAP SVMR have been taught in the prior art. Gill, (U.S. Pat. No. 6,122,150) teaches a formation in which an SyAP tri-layer is formed of two 20 A layers of Co90Fe10 separated by an 8 A layer of Ru. This tri-layer is exchange coupled to an antiferromagnetic pinning layer of 425 A of NiO. Although Gill teaches several other refinements of this structure, it is the SyAP tri-layer that exemplifies the prior art for our purposes. Huai et al. (U.S. Pat. No. 6,175,476 B) teaches the formation of a SyAP pinned layer with high resistivity and improved thermal stability by using a 4-10 A Re (Rhenium) layer rather than a Ru (Ruthenium) layer as the antiferromagnetic coupling layer. Huai also teaches an annealing method for setting the domain state of the AFM pinning layer by heating the pinning layer above its blocking temperature and then cooling it in the presence of an applied magnetic field. The applied magnetic field aligns the domain state of the adjacent pinned layer which fixes the domain state of the pinning layer. Pinarbasi (U.S. Pat. No. 6,201,671) teaches the formation of a bottom SVMR sensor (a configuration in which the AFM pinning layer is vertically below the pinned and free layers) in which an NiO AFM layer is formed on a TaO seed layer for the purpose of improving the SVMR magnetoresistance (dR/R). Pinarbasi (U.S. Pat. No. 6,208,491) teaches the formation of a capping structure for a SyAP pinned layer SVMR to improve its magnetoresistance under high temperature conditions. Finally, Pinarbasi (U.S. Pat. No. 6,208,492 B1) teaches the formation of a bilayer seed structure on which is formed an antiferromagnetic pinning layer for a SyAP pinned layer.
As magnetic storage media densities increase, the shield-to-shield thickness of the SVMR must correspondingly decrease to provide the necessary resolution of the rapid magnetic flux changes. To decrease this thickness, the SVMR stack, including all layers that contribute to its operation, must itself be made thinner. Since the thickest layer in the SVMR stack is the antiferromagnetic (AFM) pinning layer (e.g. an MnPt layer of thickness exceeding 150 A for a recorded density of 30 Gb/in2), it becomes desirable to reduce the thickness of that layer. Another reason that reducing the AFM pinning layer thickness would be advantageous, is that a portion of the sensing current necessary for sensor operation is shunted through the pinning layer. This current loss reduces the ultimate magnetoresistive sensitivity of SVMR operation because the shunted portion of the current is unaffected by resistance changes and cannot contribute to the voltage variations that are ultimately sensed. However, thinning the AFM layer will reduce the exchange bias energy (Jex) between that layer and the pinned layer. In addition, it is found that the AFM pinning layer produces another disadvantageous effect, it creates hysteresis effects (open R-H loops) in the relationship between R (magnetoresistance) and H (external magnetic field). This hysteresis is due to the AFM induced anisotropy, Hck, which leads to sensor instability. Unfortunately, when the AFM pinning layer is reduced in thickness to improve sensor resolution, the induced anisotropy is not reduced although the pinning energy is. Therefore, the hysteresis effect becomes worse.
On the other hand, a SVMR sensor for higher recording densities requires a higher pinning strength so that the smaller and more rapid external field variations can be more accurately sensed without hysteresis. We have found (and will show below), through simulations and empirical results, that AFM pinning layer thickness can, in fact, be reduced if the coupling between the two antiparallel ferromagnetic pinned layers of the SyAP, AP1 and AP2, can be improved. It is to this end that the present invention is directed.
A first object of this invention is to provide a method for forming a SVMR sensor having a thinner stack and, therefore, decreased shield-to-shield spacing.
A second object of this invention is to provide a method for forming a SVMR sensor capable of reading magnetic media with storage densities up to and exceeding 70 Gb/in2.
A third object of this invention is to provide a method for forming a SVMR sensor having an enhanced GMR ratio.
A fourth object of this invention is to provide a method for forming a SVMR sensor having improved hysteresis properties.
A fifth object of this invention is to provide a method for forming a SVMR sensor having an increased ESD threshold.
A sixth object of this invention is to provide a method for forming a SVMR sensor in a manner that provides a larger annealing window (range of annealing temperatures) than is provided by methods taught within the prior art.
A seventh object is to provide the sensor so formed by the methods of the present invention.
In accord with the objects of this invention there is provided a method for forming an SVMR sensor element having a synthetic antiferromagnetically pinned (SyAP) layer and an antiferromagnetic (AFM) pinning layer wherein the AP1/AP2 coupling between the two ferromagnetic layers is improved by the use of an ultra-thin non-magnetic coupling layer and whereby the use of an extremely thin antiferromagnetic pinning layer is thereby permitted. In this context, the AP2 layer is the pinned ferromagnetic layer closest to the AFM layer, whereas the AP1 layer is the pinned ferromagnetic layer that is closest to the Cu spacer layer that separates the antiferromagnetically pinned synthetic tri-layer from the ferromagnetic free layer. Further in accord with the objects of the present invention there is provided a method for forming a SVMR sensor element in a top spin valve configuration, since the pinning field for such a top spin valve configuration is found to be stronger than that of an equivalent bottom spin valve configuration, even though the industry trend is towards the formation of bottom spin valves. Further in accord with the objects of the present invention there is provided a method for forming a synthetic antiferromagnetically pinned (SyAP) layer for such a top spin valve configuration, wherein said SyAP layer is of the form CoFe 20 A/Ru 3 A/CoFe 15 A. Because of the high ( greater than 20 kOe) saturation field, Hs, of this SyAP layer, usual annealing methods to fix the antiferromagnetic coupling are not possible. To achieve the objects of the present invention, therefore, a low field annealing process as set forth in related patent application HT 99-011 and fully incorporated herein by reference, is applied. Yet further in accord with the objects of this invention there is provided a method for forming a SVMR sensor of the bottom spin valve type also having the advantageous properties discussed above. Said bottom spin valve type, however, cannot use a SyAP layer with a 3 A Ru spacer due to pinhole formations, so a thicker Rh spacer layer of thickness between 4-6 A is formed. Said Rh based SyAP structure has an even greater Hs than the Ru layer structure, so the low field annealing process discussed above must also be applied. The advantageousness of the present method for eliminating soft ESD damage is fully set forth in related patent application HT 00-032, which is incorporated herein in its entirety by reference.